A clutch mechanism of the above kind is adapted to be carried, through its cover plate, on a plate which constitutes the reaction plate of a clutch. The reaction plate is fixed to a first shaft, the latter being generally a driving shaft such as the crankshaft of the engine of the vehicle. A clutch friction wheel is interposed between the reaction plate and the pressure plate, the clutch friction wheel being fixed to a second shaft for rotation with the latter; this second shaft is, in general, a driven shaft such as the input shaft of the gearbox of the vehicle. A clutch release bearing is disposed around the driven shaft for controlling disengagement of the clutch, which is of the normally engaged kind. When the clutch is engaged, the pressure plate holds the clutch friction wheel against the reaction plate under the biassing effect of the peripheral portion of the diaphragm, i.e. the Belleville ring portion of the latter, so that the diaphragm bears on the primary abutment mentioned above.
A mechanism of the above kind is described in U.S. Pat. No. 4,751,991 and the corresponding French published patent application FR-2 585 424A. In that document, the crown ring is of frusto-conical form, and the weakening groove, or score line, defines a profile in the form of a trapezium which consists of a flat base between two inclined side flanks. That arrangement is satisfactory to the extent that it enables the gripping force to be obtained very precisely by virtue of the weakening groove. However, it is desirable to obtain even better control of the gripping force applied by the diaphragm between its primary and secondary abutments, in order to reduce friction effects.